1. Field of the Invention
The present invention relates to a laminated composition comprising a thermoplastic polymer having degradability in the natural environment and a regenerated cellulose film (cellophane), paper, leather, cloth or fiber. More particularly, the invention relates to a degradable, laminated composition comprising a polymer essentially consisting of polylactic acid or a lactic acid-hydroxycarboxylic acid copolymer and cellophane, paper, leather, cloth or fiber. The laminated composition can be used as moisture-proof packaging papers, decorative papers, containers for milk and juice, and other packaging materials. These materials can be readily degraded after use in the natural environment.
2. Related Art of the Invention
The amount of packaging plastics has recently been increased in view of beauty, hygiene, packaging and transport of commodities. As a result of such trend, the amount of refuse discarded from households and factories has rapidly increased and the shortage of burial site has caused a serious problem around cities.
A regenerated cellulose film called cellophane, paper, leather, natural fiber or cloth prepared from natural fibers has degradability in the natural environment, is degraded in a long period though buried under the ground, and does not develop hazardous gas in the case of incineration. Some materials are recovered and reused in view of resource protection. Thus these degradable materials are used for disposable wrapping papers, containers and other various packaging materials in place of difficultly degradable plastics and are expected to have increased uses in the future.
However, a laminated composition comprising a thermo-plastic polymer having degradability in the natural environment and a regenerated cellulose film (cellophane), paper, leather, cloth or fiber has not yet been known.
Cellophane has a substantial much demand as a packaging material which is excellent in transparency, impermeability to gases, easy printability and gloss. However, cellophane is inferior in moisture-proofness, low-temperature resistance, strength and heat-sealing characteristic, and hence is usually used by being laminated with a synthetic polymer in order to compensate for these drawbacks. Packaging papers and paper containers have low strength and poor water resistance and thus paper packs and paper cups for milk and juices are prepared from laminated paper obtained by laminating a polymer and paper. Book covers and dressing cases are generally prepared from laminated paper having a smooth surface and gloss in order to satisfy the desire of the consumer for beauty. Leather, cloth and fiber are also used for wrapping papers, containers and other various packaging materials as principal or secondary components and have similar disadvantages.
Polymers for use in these uses have been polyolefins such as polyethylene and polypropylene, and paper-coating polyester. These polymers, however, have almost no degradability in the natural environment. When laminates are abandoned and buried under the ground, these polymers greatly lower the degradation rate of cellophane, paper, leather and cloth which are substantially degradable in the natural environment. A large amount of heat generated in the incineration of these polymers leads to problems such as damage to the furnace. When the laminates are recovered, problems occur in separating these polymers.
Polylactic acid and lactic acid copolymers have recently been known as thermoplastic and biodegradable polymers. These lactic acid polymers can be completely biodegraded within several months to a year in an animal body. When the polymers are placed in soil or sea water, the polymers start to decompose within several weeks in a moist environment and disappear in about a year. The degradation products of these polymers are lactic acid, carbon dioxide and water, and all of these compounds are nontoxic.
U.S. Pat. Nos. 1,995,970, 2,362,511, and 2,683,136 have disclosed a polymerization process of lactic acid. U.S. Pat. Nos. 3,636,956 and 3,797,499 have disclosed a process for copolymerizing lactic acid and glycolic acid. Polymers of lactic acid are usually prepared from a cyclic dimer of lactic acid which is called lactide. In the copolymerization of lactic acid, lactide and glycolide, i.e., a dimer of glycolic acid, are mixed and ring-opening polymerization is carried out. When the polymer is directly prepared through dehydrating condensation from lactic acid or a mixture of lactic acid and glycolic acid, a high molecular weight polymer cannot be obtained even though the reaction is carried out for a long time. On the other hand, the ring-opening polymerization of lactide or of a mixture of lactide and glycolide can provide a high molecular weight straight-chain polyester.